冬季一次引发华北暴雪的低涡涡度分析

Analysis of Vorticity during Vortex Producing Snowstorm in North China in Winter

利用NCEP FNL 1°×1°再分析资料和WRF模式,模拟了2010年1月2～3日我国华北地区的一次由涡旋造成的冬季降雪过程,并采用位涡和涡度方程对引发暴雪的涡旋发展机制进行了诊断分析。结果表明,这次降雪过程中,对流层中层高空浅槽东移、加深及发展,并引导低空和地面系统自西向东移动,高空位涡的下传强迫加强了对流层中低层涡旋的发展。平均通量和涡旋区域的辐合、辐散作用对涡旋涡度的增长贡献最大,扰动通量和类倾斜项的作用较小。在中层涡旋成熟期,环境场的风速小于中层涡旋的移动速度时,环境场相对于涡旋区域为辐散,涡旋涡度减小;当环境场风速大于涡旋的移动速度时,环境场相对于涡旋区域为辐合,涡旋涡度增加。在涡旋衰减期,向涡旋外输送的绝对涡度通量使得涡旋涡度逐渐减弱。这次过程中,高空位涡强迫、低空辐合和涡旋边界平均气流对扰动涡度的输送是涡旋发展的主要机制。

The snowstorm occurred in North China on 2~3 January 2010 was simulated using WRF model.And potential vorticity and vorticity equation were used to diagnose the developing mechanism of the vortex producing heavy snowfall process in winter.The results showed that,firstly,the weak trough in middle troposphere leaded to lower and surface systems moving from west to east.Secondly,potential vorticity from upper-level intensified development of vortex in middle-lower troposphere.Thirdly,the mean flux and stretching term of the integrated vorticity equation was the main source of vorticity,but eddy flux and like-tilt term was relatively weak.During the mature period of vortex,the vorticity decreased because of the divergence of environmental circulation to vortex area while the vortex moved faster than surrounding air,and to the contrary,vorticity increased for the convergence between environmental circulation and vortex area.During the dissipating period,the vorticity decreased since the loss of the absolute vorticity flux.On the basis of all diagnosis,the PV forcing in upper-troposphere,convergence in lower-troposphere and the vorticity produced by environmental circulation are main mechanisms of vorticity developing.